Pipe system for carrying a medium through their inside will over time accumulate unwanted materials or impurities on its walls, which will reduce its fluid transport capacity. Likewise, such unwanted materials or impurities will or can contaminate the fluid to be transported through the pipe system, for example oil, which in turn can be detrimental for the functioning of valves that are connected to the pipes. Such unwanted materials or impurities will therefore result in reduced operational capacity either directly through reduced flow or indirectly due to the efforts, time and resources required to maintain the pipe system and attached valves at an acceptable level of cleanliness.
Particular pipe systems are hydraulic systems such as so called umbilical cords used in offshore industry. Installations can vary in length, for example several kilometers long, and connect equipment placed at industrial sites, offshore installations, or sub-sea installations, further increasing the complexity of operating and maintaining the installation at a certain and required level of cleanliness.
In the particular case where the hydraulic pipe system uses an oil-based fluid, the impurities or dirt that accumulated on the inside of the hydraulic pipe system is usually wax or grease. Wax or grease is typically also present in new pipes or coils and should be removed before installation. Otherwise, impurities from the fluid, for example oil, being conducted through the pipe will stick to the wax or grease. The impurities may also be transported into valves used in deep-sea installations with the risk of malfunctioning, why a proper cleaning before installation of the pipes in such systems is important.
In order to clean pipe systems, it has been proposed to use turbulent flow in such pipes with a cleaning and flushing liquid. The turbulent flow assists in loosening contaminants that adhere to the inner wall of the pipes and flush away the contaminants. In the UK patent application GB2323421 by Thomsen, assigned to Ocean Team Scandinavia, a system is disclosed with fluid pipes are cleaned with a pulsated flow. In order to obtain a turbulent flow, a Reynolds number of at least 2300 or at least 3000 is mentioned
When narrow pipe systems get very long, the pressure drop of the cleaning fluid throughout the pipe results in loss of turbulent flow, because the speed of the flow cannot be kept high enough. This problem is discussed in U.S. Pat. No. 5,007,444 by Sundholm; the pressure drop in pipes that are longer than 200 m and with a narrow lumen of 10 mm prevent a flushing speed that creates a turbulent flow, because the pressure required at the entrance of the tube to compensate for the pressure loss along the pipe and for create the necessary flow speed would exceed the pressure that the pipes typically withstand. As a solution to this problem. U.S. Pat. No. 5,007,444 proposes filling the pipe system with flushing liquid as well as gas such that number of portions of flushing liquid in the pipe is separated by gas portions. The gas in the alternating columns of oil and gas is compressed for subsequent expansion when a valve is opened in order to create a forceful flushing pulse through the pipe system.
However, despite various suggestions for long, narrow pipes, proper cleaning is a problem that is still not solved in a satisfactory way, such that there is a need for further improvement in the art.
Often, oil is used as a cleaning and flushing liquid. For cleaning and flushing of pipe systems, heat exchangers, condensers and catalysers, liquid carbon dioxide (LCO2) or supercritical carbon dioxide (SCCO2) has been proposed in German utility model DE20113516U1 by Kipp. As illustrated in the figures of DE20113516U1, LCO2 or SCCO2 is led into the bottom of a heat exchanger and extracted through a top valve before being filtered as gas and recirculated. In DE20113516U1, no details are given with respect to flow speed or pressure other than the pressure and temperature necessary to keep the carbon dioxide (CO2) in a liquid or supercritical state. It is explained that the LCO2 and the SCCO2 would loosen the contamination from the inner walls. Although, the description states that contaminants are removed from the system and then filtered, no concrete considerations appear to have been taken whether the contaminants can actually be removed from the system in practice. Calculations in this regard will be explained in more detail below, indicating that the system in DE20113516U1 cannot to be expected to work in practice.
All in all there is still a need for improvements in the field of pipe cleaning.
Rinsing cavities with supercritical CO2 is disclosed in US2009/0107523 by Zorn. CO2 gas as a flushing in submarines is disclosed in European patent application EP2151377 by Krummerich et al. Use of hydrocarbon fluids for cleaning a chemical or hydrocarbon processing plant is disclosed in WO2003/103863. Substrate cleaning with SCCO2 is disclosed in WO2003/046065 by Bertram et al.